This research investigates at what points in the metabolism of nucleic acids the two teratogens 5-fluorouracil (5-FU) and 5-fluorodeoxyuridine (5-FUdR) interact with Strong's luxoid gene (1st) and its plus or minus modifiers to disturb limb development in two strains of mice selected for low or high expression of 1st's effects on the limb skeleton. The research also investigates the bases in nucleic acid metabolism of the potentiating effects of uracil on the teratogenic effects of 5-FU and the protective effects of thymidine on the teratogenic effects of 5-FUdR. Also studied is the question whether Strong's luxoid gene or its modifiers have any effect on nucleic acid metabolism. Radioactive tracers are used to study in both pregnant females and their embryos aspects of nucleic acid metabolism which may be affected by 1st and its modifiers but which are known to be affected by the two teratogens.